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Aerobatics with Oil Accumulator

Jared_Solomon

Well Known Member
Hi All,

I've been doing a lot of aerobatics lately and recently installed a Half Raven and an Accusump 2qt system on my RV-14A. So far, I pretty disappointed with the Accusump's performance and I'm wondering if i'm expecting too much from it. I track all of my engine data and debrief my acro flights with CloudAhoy. Before the Accusump, when performing stall turns, my oil pressure would drop to about 37psi. I installed the Accusump thinking it would keep the oil pressure from dropping so low during my brief zero G section of the maneuver. Unfortunately after analyzing the data the oil pressure diminished at the same rate (down to 37psi) both with and without the Accusump. I've read other's mention in previous threads that it takes 5-7 seconds for their oil PSI to fall to 25psi with a 1.5qt Accumulator. At what PSI does the pressure drop start to slow down for most of you? I would guess that without the accumulator my engine would take about 5-7 seconds to fall to 25 PSI anyway...at least that's what I've been seeing during my zero G stall turns. What am I missing? I would like to get a better idea of the expected performance of the accumulator before I start to debug and assume there is something wrong with either it or the installation.

Some details:
  • Accusump is precharged to 10psi
  • Accusump is connected via a T-fitting on the exit port of the oil cooler
  • Accusump pressure gauge reads 85psi at idle
  • The Accusump has no valve (i.e. EPC valve) connected
 
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I would like to see the pressure numbers vs time vs attitude & g loading for a full inverted system compared to the 2 systems you have used(stock sump & your accusump accumulator) Vertical down with no wing loading presents a no oil to the pump situation regardless of what system is used, at least beyond a few seconds.

Perhaps Gash has an engine monitor that records to a card on his Extra.

I appreciate your report and look forward to others data.
 
Poor performance with oil accumulator

First, the necessary disclaimer that I have no experience whatsoever with an oil accumulator, though I am an engineer which avails me with enough knowledge that may occasionally substitute for real experience….At a charge of only 10 PSI, I would not expect a 2 quarter accumulator to be able to push very much oil at all back into the System at oil pressures running that high (25 to 37 psi) compared to 10 PSI. I would think that if you double or triple the pressure, you are more likely to see the accumulator initially kick into action at higher pressures. Even if one’s only concern is pumping oil at the very lowest pressures, 10 PSI I would think would just barely be enough as a guess, except if using as a pre oiler at start up.

Again, I have no practical experience whatsoever…Only theoretical. Note also, at higher accumulator pressures, the amount of oil the accumulator can supply naturally goes down because there is less oil on the oil side of the accumulator in the first place before the low pressure event in the system.

The peanut gallery…
 
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I’ve run accusumps in race cars for years. Most with the epc valve. This electric valve opens fully at a set pressure. It’ll let oil charge the sump when closed but not release it. Meaning if the sump is charged at 85 psi, once the pressure drops to 35 psi (or whatever it’s set to) it’ll fully open and dump an 85psi shot of oil into the system. You dash gauge would reflect this surge.
On those I’ve run without the epc, the sump just serves as a reservoir that reflects system pressure. The oil flows in/out as your oil pump pressurizes the system. It’ll help some if oil pickup becomes uncovered but not so you could see it on gauge. Putting more pressure in the sump will just limit the amount of oil it’ll hold. A 2qt sump won’t hold 2 quarts… My experience anyway with road race cars under turns and braking.
 
Accumulator operation is tricky. Obviously the pressure on the precharge side has to be higher than the oil pressure to get the piston to move. As it does it lowers the precharge pressure due to the increased volume. The air in the accumulator also cools because of the work it is doing by moving the piston and lowers the pressure further.

The rule of thumb is to charge to 80-90 percent of the minimum acceptable oil pressure. At the optimum pressure you still only get to use about 60-70 percent of the accumulator volume. Your accumulator is delivering oil but just at a lower pressure than you had hoped. Be careful increasing the precharge pressure above what the manufacturer recommends.

Here is a calculator that you can try simulations with.

https://www.accumulators.com/tool/calculate-usable-volume/
 
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Accumulator operation is tricky. Obviously the pressure on the precharge side has to be higher than the oil pressure to get the piston to move. As it does it lowers the precharge pressure due to the increased volume. The air in the accumulator also cools because of the work it is doing by moving the piston and lowers the pressure further.

The rule of thumb is to charge to 80-90 percent of the minimum acceptable oil pressure. At the optimum pressure you still only get to use about 60-70 percent of the accumulator volume. Your accumulator is delivering oil but just at a lower pressure than you had hoped.

Here is a calculator that you can try simulations with.

https://www.accumulators.com/tool/calculate-usable-volume/

This aligns with industry standards; pre-charge 80-90% system press when primary purpose is make-up power (pressure). ~ 50% when the primary purpose is pressure pulsation dampening, others. Of course, these numbers could be anything depending on specific application but since weight is such a big consideration, a low pre-charge press is very suspicious. For my benefit, do these accumulators have a bladder?
 
Thanks. I would have lost that bet. For my benefit, how do you verify no cross over; the dry side is staying dry and charged?
 
Do you have canted balls?
There is no reason for the oil pressure to drop during a stall turn, the oil system must transition to the inverted feed (top rear of crankcase) in the up vertical and to the regular feed (fwd end of sump) in the down vertical. That is done by mounting the 3 ball transfer valve at an angle, so not directly bolted to the firewall. For example 4 washers on the lower mounting bolts is often enough.
For steam powered aircraft it is not unusual to hear the engine note drop in the up vertical as the oil pressure diminishes and the counter-weighted prop goes coarse. Angling the balls prevents that, no idea of the actual op values just that the rpm doesn't drop.
Pete
 
I'll just chime in my experience with the 2qt accusump operation as well. Empty precharge level is set to 10psi, like their instructions say. With the EPC valve, the accumulator would charge to >150psi with the engine making 70psi on the gauge. My accumulator is T'd into the oil cooler feed line. I eventually had to bypass the pressure sensor on the EPC valve so that the valve is either open or close based on switch position. Now the accumulator holds ~110 psi with the engine warmed up and running 60-70psi. The accumulator pressures have been verified with both the garmin 150psi gauge and a mechanical gauge on the air side of the accumulator. I bought the manual valve for the accumulator, but haven't installed it yet. The EPC valve builds dangerous amount of pressure in the accumulator when wired per the accusump instructions. Bypassing the pressure switch basically turns it into a heavy manual valve, so I'm going to save myself the 2ish pounds and swap them out.
 
I'll just chime in my experience with the 2qt accusump operation as well. Empty precharge level is set to 10psi, like their instructions say. With the EPC valve, the accumulator would charge to >150psi with the engine making 70psi on the gauge. My accumulator is T'd into the oil cooler feed line. I eventually had to bypass the pressure sensor on the EPC valve so that the valve is either open or close based on switch position. Now the accumulator holds ~110 psi with the engine warmed up and running 60-70psi. The accumulator pressures have been verified with both the garmin 150psi gauge and a mechanical gauge on the air side of the accumulator. I bought the manual valve for the accumulator, but haven't installed it yet. The EPC valve builds dangerous amount of pressure in the accumulator when wired per the accusump instructions. Bypassing the pressure switch basically turns it into a heavy manual valve, so I'm going to save myself the 2ish pounds and swap them out.

Hmm
If the oil pressure and precharge pressure aren’t about the same with the engine running something is blocked or stuck. The piston should move until the pressures are the same.
 
ECP valve is potential trouble.

I’ve run accusumps in race cars for years. Most with the epc valve. This electric valve opens fully at a set pressure. It’ll let oil charge the sump when closed but not release it. Meaning if the sump is charged at 85 psi, once the pressure drops to 35 psi (or whatever it’s set to) it’ll fully open and dump an 85psi shot of oil into the system. You dash gauge would reflect this surge.
On those I’ve run without the epc, the sump just serves as a reservoir that reflects system pressure. The oil flows in/out as your oil pump pressurizes the system. It’ll help some if oil pickup becomes uncovered but not so you could see it on gauge. Putting more pressure in the sump will just limit the amount of oil it’ll hold. A 2qt sump won’t hold 2 quarts… My experience anyway with road race cars under turns and braking.

I'll just chime in my experience with the 2qt accusump operation as well. Empty precharge level is set to 10psi, like their instructions say. With the EPC valve, the accumulator would charge to >150psi with the engine making 70psi on the gauge. My accumulator is T'd into the oil cooler feed line. I eventually had to bypass the pressure sensor on the EPC valve so that the valve is either open or close based on switch position. Now the accumulator holds ~110 psi with the engine warmed up and running 60-70psi. The accumulator pressures have been verified with both the garmin 150psi gauge and a mechanical gauge on the air side of the accumulator. I bought the manual valve for the accumulator, but haven't installed it yet. The EPC valve builds dangerous amount of pressure in the accumulator when wired per the accusump instructions. Bypassing the pressure switch basically turns it into a heavy manual valve, so I'm going to save myself the 2ish pounds and swap them out.

This ECP valve is responsible for multiple failures of the Accusump containers. When electrically off, it will act as a check valve, BAD. If shut off under oil pressure, then it captures a peak even if transient. Then, the oil expansion with cowl temps will exceed the container burst pressure. Failures have been progressive, not the result of a single event. The piston has a cavity that allows the piston to hit the housing and become solid. That could be a reason for panel pressure and "charge gage" pressure to be different. The 10 psi allows the max volume to be available. More precharge would help one issue but reduce the functionality.

I noticed my charge pressure was elevated and sucked out some oil from the air side. In theory there is no pressure across the piston o-ring, but when solid, there is pressure if the piston hits.

Theory is good if one knows the real internal geometry. Inaccurate assumptions lead to errors, it is not the "theory". I made a model with the actual dimensions to discover the piston would hit the end cap.

I can not answer the OP question and have no data. Ron Schreck took his accumulator off and installed an inverted system. I too am considering removal.
 
Hmm
If the oil pressure and precharge pressure aren’t about the same with the engine running something is blocked or stuck. The piston should move until the pressures are the same.

With the manual valve I totally agree. With the ECP valve it just isn't the case. I wouldn't suggest the ECP valve to anyone. It's heavy and builds unsafe levels of pressure. Just put a manual valve in and you'll be better off.
 
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